The present invention relates to a power transmission for an automotive vehicle, and more particularly to a power transmission for a four-wheel drive vehicle which is adapted to a prime mover of the vehicle to transmit therethrough a driving torque of the prime mover to a first drive shaft for front-wheel drive and to a second drive shaft for rear-wheel drive.
As one of conventional power transmissions of this kind, there has been proposed a power transmission which includes a transmission casing arranged to be secured to a prime mover of the vehicle, a change-speed gearing mounted within the transmission casing and drivingly connected to an output shaft of the prime mover, and an intermediate differential unit mounted within the transmission casing and drivingly connected to an output shaft of the change-speed gearing to be applied with a driving torque from the prime mover through the change-speed gearing for transmitting the driving torque to a first drive shaft for front-wheel drive and to a second drive shaft for rear-wheel drive. The intermediate differential unit includes a differential raging rotatably mounted within the transmission casing, a pair of pinion gears rotatably mounted within the differential casing, and a pair of side gears rotatably mounted within the differential casing and being in meshing engagement with the pinion gears and respectively connected to the first and second drive shafts.
In such a conventional intermediate differential unit as described above, the side gears are formed in general in the same diameter to equally transmit the driving torque to the first and second drive shafts. It is, however, noted that the distribution ratio of the driving torque to the drive shafts should be changed in accordance with a road condition to ensure stable maneuverability of the vehicle. For example, if the distribution ratio of the driving torque is determined to ensure stable maneuverability of the vehicle on a road of low frictional coefficient, there will occur understeer of the vehicle on a road of high frictional coefficient. If the distribution ratio of the driving torque is determined to ensure stable maneuverability of the vehicle on a road of high frictional coefficient, traction of the vehicle will become insufficient on a road of low frictional coefficient. For the above reason, an auxiliary transmission is adpated to the intermediate differential unit to change the distribution ratio of the driving torque in accordance with a road condition. Such arrangement of the auxiliary transmission, however, results in increase of the size and weight of the power transmission assembly.